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Microbial Nutrition and Growth
K.SAI KUMAR
PHARM.D
Some of the information relating to microbial nutrition and growth has been presented in
previous lectures or in laboratory sessions. This information will be included here as a
review.
Nutritional Categories:
Microorganisms (and macroscopic forms also) can be divided into four nutritional
categories based on the sources they use for energy and carbon. The four categories are:
1) Chemoheterotrophs = Organisms requiring preformed organic compounds for both
their energy and carbon requirements. These organisms may also be referred to as
chemoorganotrophs.
2) Chemoautotrophs = Organisms that use chemicals for energy, but are capable of using
inorganic compounds (e.g., CO2, HCO3-, etc.) for carbon. If these organisms use only
inorganic chemicals to meet their nutritional needs, they may be called
chemolithotrophs.
3) Photoheterotrophs = Organisms that use light energy and require preformed organic
compounds as their source of carbon.
4) Photoautotrophs = Organisms that use light energy and are capable of using inorganic
compounds (e.g., CO2, HCO3-, etc.) for carbon.
Animals (including humans), fungi, protozoa and many types of prokaryotic organisms
are chemoheterotrophs. Of these, some are saprotrophs, some parasites, some
hypotrophs and many are free-living organisms consuming other life forms as food
materials. Free-living protozoa may be considered predators (carnivores) or herbivores,
depending on the prey they consume, but many of these are omnivorous.
Green plants, algae, cyanobacteria and some other types of prokaryotic organisms are
photoautotrophs. These are ecologically considered as producers because they provide
much of the organic material consumed by various different types of chemoheterotrophs.
Most photoheterotrophs are prokaryotic, but because some single-celled eukaryotic forms
are capable of switching between nutritional categories, they might function as
photoheterotrophs for short periods of time. All chemoautotrophs are prokaryotic.
Culture media:
Most of the microorganisms grown under laboratory conditions are chemoheterotrophs.
This is particularly true in clinical laboratories where human pathogens are of primary
interest. Many chemoheterotrophs can be grown on or in mixtures of materials designed
to support their metabolic processes. Mixtures of materials providing all the nutrients
needed to grow microorganisms in vitro, i.e., in artificial containers, are called culture
media (singular = medium). When considering the nutrients necessary for growth, it is
useful to consider the composition of protoplasm and the elements incorporated into
various compounds. Some common nutrients are listed below.
1) Carbon – Carbon is essential for the synthesis of all organic compounds,
carbohydrates, proteins, lipids and nucleic acids. Though autotrophs can obtain
carbon (carbon dioxide) from the air, heterotrophs cannot, and must have this element
provided to them in the culture medium. Carbon is often provided in carbohydrate
form (glucose, lactose, mannitol, etc.) or as protein or protein breakdown products
(peptones, peptides or proteoses). Media without carbon can be used to grow
cyanobacteria.
2) Nitrogen – Nitrogen is essential for the synthesis of proteins, nucleic acids and some
carbohydrates, e.g., glucosamine. Since proteins and protein breakdown products
provide both carbon and nitrogen, these are common ingredients in culture media.
Some prokaryotic organisms can obtain nitrogen from the air (molecular nitrogen =
N2), and are called nitrogen-fixing organisms. Bacteria with this capability can grow
readily on nitrogen-free media.
3) Minerals – Many of the elements incorporated into organic compounds are minerals
such as sulfur, phosphorous, iron, calcium, magnesium, iodine, manganese and
copper. Sulfur is incorporated into certain amino acids, so is essential to protein
formation. Phosphorous is used for the synthesis of phospholipids, nucleic acids and a
variety of nucleotide derivatives such as coenzymes (NAD, FAD, NADP), and high
energy compounds (ATP, GTP, etc.). Iron and copper often serve as prosthetic groups
in enzymes with quaternary structure, and calcium, magnesium and manganese often
function as cofactors essential to enzyme activity. Electrolytes such as NaCl, KCl and
CaCl2, dissociate into ions involved in maintaining membrane potentials, activating
contractile elements (e.g., microfilaments) and other cellular functions. Iodine is
incorporated into various organic compounds.
4) Water – Though not usually considered as a nutrient, water is essential to metabolism,
and is incorporated into organic compounds (as H+ and OH-) during hydrolysis
reactions. The amount of water present relative to solute materials also influences
tonicity, making environments isotonic, hypotonic or hypertonic. Though bacteria are
not damaged by osmosis when placed in hypotonic environments (because they are
equipped with walls), the nutrient levels in hypotonic media tend to be low, and
growth may be slow. Hypertonic environments tend to inhibit growth because they
cause water to exit cells and metabolism to slow or stop. For this reason, salt and sugar
are often used to preserve food materials. Since high salt environments tend to occur
naturally in various places, e.g., along rocky ocean shores, in deserts, on skin surfaces
and inside nasal passages, many microorganisms have adapted to these environments.
These salt-loving organisms are called halophiles.
5) Buffers – Some of the minerals listed above are also incorporated into buffers, i.e.,
substances that resist pH change when the acidic or alkaline waste products of various
microorganisms are released into media. K2HPO4, KH2PO4 and CaCO3 are examples
of materials often incorporated into buffer systems. Culture media without buffers
often contain pH indicators, i.e., substances that change color in response to changes in
pH (the acidity or alkalinity of the environment). Phenol red, bromothymol blue and
bromocresol purple are pH indicators commonly used in culture media.
6) Other – Some organisms will only grow when provided with specific growth factors
such as vitamins, amino acids, cells, tissue fragments or other materials. Such media
are called enriched media and are used to grow fastidious microorganisms (picky
eaters). Bacteria in the genera Streptococcus and Neisseria tend to be fastidious and are
often grown on enriched media such as blood agar or chocolate agar.
As demonstrated in the laboratory, culture media may be categorized in a variety of ways.
Broth media are liquid in form and when incubated on a moving surface such as an orbital
shaker, allow all organisms present to maintain contact with the nutrients provided. Solid
media contain some type of solidifying agent in addition to the nutrients present. Though
a variety of solidifying agents could be used, agar (a polysaccharide) is the material most
commonly used to change broth media into solid media. Agar is an ideal solidifying agent
for culture media because most microorganisms do not catabolize it (break it down).
Unlike those containing gelatin or starch, solidifying agents catabolized by many different
types of microorganisms, media made with agar will remain solid as microbial cultures
grow. Another advantage of agar is its response to temperature changes. Media made
with agar will remain solid at relatively high incubation temperatures (considerably above
37o C) while media made with gelatin will liquefy. Agar is made from algae in the phylum
Rhodophyta.
Culture media containing agar are prepared as liquids and poured into various containers
where they are allowed to solidify. Agar deeps, slants/slopes and plates are commonly
used in our laboratory. Agar plates provide broad surfaces where cultures can be streaked
and colonies isolated to check for purity. Agar slants and deeps are often used for
enzymatic testing, but can also be used for long-term storage if the culture tubes used are
equipped with screw-on caps.
Factors influencing microbial growth:
Many factors influencing microbial growth were described in association with criteria
used in microbial classification. These same factors can also be associated with microbial
control, and will be described in that context later. Some factors known to influence
microbial growth include:
1) Gas requirements – Microorganisms can be categorized as obligate aerobes,
facultative anaerobes, obligate anaerobes or microaerophiles based on their gas
requirements. Obligately aerobic organisms have respiratory/oxidative metabolic
processes and use molecular oxygen (O2) as a final electron acceptor. Aerobic bacteria
often grow near the surface in broth media and sometimes form pellicles or skin-like
coverings at the medium surface. Facultatively anaerobic organisms can often use
either respiratory or fermentative metabolic processes and switch back and forth
between the two as their environments permit. Facultative organisms will grow
throughout broth media if they are motile, or if agitated during incubation; but nonmotile forms will often form a mass of sediment at the bottom of a stationary tube.
Obligately anaerobic forms may also form sediments on tube bottoms, but typically
require special containment or anaerobic media for growth. Many of the organisms
inhabiting the Winogradsky columns and window are obligately anaerobic.
2) Temperature – Organisms can be categorized as psychrophiles, mesophiles,
thermophiles or hyperthermophiles based on the temperatures they prefer for growth.
Bacteria as a group can inhabit environments with extreme temperature differences,
e.g., the freezing temperatures of polar ice Vs the near boiling temperatures of hot
springs or deep-sea thermal vents; but individual species typically have relatively
narrow temperature ranges. Enzyme activity and therefore metabolism is dependent
on temperature, and enzymes vary considerably with respect to their temperature
optimums. Though most of the bacteria grown in our laboratory are mesophiles,
placing cultures in a 37o C incubator will not necessarily increase their growth rate.
Many organisms associated with air plates prefer temperatures below this.
3) pH requirements – The amount of acidic or alkaline material present in culture media
can significantly influence microbial growth. Most media used in our laboratory are
initially pH-adjusted to be near neutral (pH = 6.8 – 7.2), particularly if they contain pH
indicators. Many bacteria can tolerate slightly acidic environments (shifts toward the
low end of the pH scale) more readily than they can alkaline environments (shifts
toward the high end of the pH scale). Some organisms, e.g., Ferroplasma, grow best in
acidic environments and can be considered as acidophiles. Although bacteria can
form both acidic and alkaline end products through their metabolic processes, acidic
products tend to stay in the media and have a greater influence on pH (alkaline end
products such as amines and ammonia are volatile and escape into the air). Organic
acids (e.g., lactic acid and acetic acid) formed by fermentative microorganisms are used
extensively in food preservation (cheese, yogurt, pickles, sauerkraut and silage),
because high levels of acid inhibit the growth of organisms likely to cause spoilage.
4) Osmotic pressure requirements – The effective osmotic pressure or tonicity of an
environment can influence growth by causing water to either enter or exit cells through
osmosis. Hypotonic environments do not damage organisms equipped with cell walls
(e.g., fungi, algae and bacteria), because walls prevent these cells from taking on excess
water and blowing up. Hypertonic environments cause water to exit cells, and will
often inhibit metabolism and growth. Since high salt environments occur naturally in
various places, many microorganisms have adapted to these. Some salt-loving
organisms or halophiles can only grow in hypertonic environments containing high
levels of salt.
5) Symbiosis – In natural environments, the growth of microorganisms is significantly
influenced by interactions with other organisms. Relationships are often subtle and
many have yet to be investigated or are not thoroughly understood. A few examples
have been described earlier, and are included here as reminders. Symbiotic
relationships between bacteria in the genera Rhizobium and Photobacterium and plants
or fish/squid, respectively, are mutually beneficial to all organisms concerned, i.e., are
mutualistic. Bacteria, protozoa and fungi living inside the gastrointestinal tracts of
animals (including humans) gain nutrients provided to them by their hosts, but can
also provide animals with nutrients they could otherwise not obtain. Maintaining
balance between the various populations present is essential to good health. Parasites
such as flatworms, roundworms and various arthropods take nutrients from animal
hosts, but generally give nothing in return. Fungi form haustoria or mycorrhizae
when interacting with plant hosts, form antibiotics that inhibit bacterial growth and
live symbiotically with algae or cyanobacteria within lichens. Ecologically speaking, all
symbiotic relationships are probably mutualistic, because even parasites and
pathogens benefit their hosts through population control, a concept foreign to many
humans.
When microorganisms are grown in vitro, symbiotic relationships are limited, but
some can still be observed. On agar plates exposed to air or soil, mixed populations of
fungi and bacteria will sometimes interact in ways visible to the naked eye. Clear areas
or zones of inhibition are often formed by colonies producing chemicals (antibiotics)
inhibiting the growth of others. When the effects of antibiotics are synergistic, i.e.,
greater when combined, this is also visible. Fungi will often grow on nitrogen-free
media if nitrogen-fixing bacteria are already present. Though fungi cannot fix
nitrogen, they can and do use by-products formed by bacteria. On blood agar plates,
hemolysis reactions can be enhanced by the combined action of different bacteria, as
demonstrated by the CAMP-reaction. Organisms with this ability can also cause more
severe damage when interacting together inside a host.
Microbial Growth:
When applied to microorganisms, the term growth refers to an increase in cell number
rather than to an increase in the size of an individual organism. When bacteria are placed
on the surface of a solid medium such as nutrient agar within a Petri plate, each living cell
in contact with the nutrients available has the potential to reproduce itself and grow into a
mass visible to the naked eye. The visible mass is called a colony, and though variable in
morphology, will often appear as a circular form with an entire margin, convex elevation,
smooth-shiny surface texture, opaque optical character and pigment varying from white to
brightly-colored (yellow, pink, purple, brown, etc.).
Like many single-celled eukaryotic organisms, bacteria typically reproduce themselves by
means of an asexual process called binary fission during which one cell divides into two.
Unlike eukaryotes, bacteria do not undergo mitosis. Prokaryotic cells do not form
microtubules, centrioles or a spindle apparatus, so the separation of chromosomes requires
an alternative mechanism. Though fission typically proceeds as a more-or-less continuous
process, it can be broken into a number of steps or phases as described below.
1) Replication of DNA (and other essential cellular components) – Before it can divide
itself into two parts, a cell must replicate its genetic information (DNA). This insures
that each new cell formed will contain the information necessary to function as a
separate organism. The details of DNA replication will be presented later; but
replication basically involves the separation of the two, nucleotide strands present in
each DNA molecule, and the formation of new complimentary strands in association
with each one. Additional cellular components such as ribosomes and enzymes are
also made.
2) Elongation – Though bacteria do not grow extensively as individuals, each cell does
undergo some degree of elongation before dividing into two parts. Since
peptidoglycan is not elastic, i.e., does not stretch, elongation requires a partial
decomposition of this rigid wall material and the deposition of new peptidoglycan.
(The process is somewhat like the growth of long bones in association with epipheseal
plates.) Decomposition typically occurs in several places along the long axes of rodshaped cells (bacilli), or in one centrally located (equatorial) region of each spherical
cell.
During elongation, the chromosomes are separated. Regions of each chromosome are
attached to the cell membrane or cytoplasmic membrane (recall membranous folds
called mesosomes), and as the cell elongates, the chromosomes are pulled or pushed
apart.
3) Septum formation – Following elongation, the cell membrane folds inward across the
long axis of the cell, until it forms a two-layered, membranous septum separating the
cytoplasm into two parts. As in eukaryotic cells, this inward folding or pinching of the
cell membrane involves microfilaments.
4) Deposition of new wall within the septum – Once the septum has been formed; new
layers of peptidoglycan are deposited between the membrane layers or within the
septum. This effectively separates the cell into two parts. Many of the diplobacilli
observed in bacterial smears are cells at this stage of the fission process.
5) Physical separation – When sufficient wall material has been deposited, the two cells
can break away from one another and exist as separate individuals; however, physical
separation does not always occur. Cells with a characteristic arrangement, e.g.,
diplococci, streptococci, streptobacilli, etc. are examples of those maintaining
connections following fission. If cells do not physically separate, their arrangement is
influenced by the orientation of the fission plane. Chains of cells (streptococci or
streptobacilli) will be formed if the fission plane always has the same orientation.
Tetrads, sarcinae and staphylococci will form if the orientation of the fission plane
changes between fission cycles.
Population Dynamics in a Batch Culture:
Bacteria grown in a broth medium within a closed container such as a tube, flask or bottle
(i.e., grown in vitro), form a batch culture. Under batch culture conditions, the population
can receive no additional nutrients, and most metabolic wastes will stay within the
medium.
A population of bacteria grown in a batch culture will typically undergo changes in
density (number of cells per volume) that when plotted against time will form a
predictable growth curve. The phases of this curve can change somewhat with respect to
time and cell density, but the overall pattern is similar for all populations. The growth
curve and the phases represented are described below. Note that ordinate values are
represented on a logarithmic scale.
1. Lag phase – The lag phase is represented by a straight, horizontal line of variable
length. During this phase of growth, the cells present are increasing in DNA content,
metabolic activity, size and dry weight, but the population is experiencing no increase
in cell number. The cells are "gearing up" for growth and completing some of the
steps involved in fission, but physical separation has not yet occurred. The number of
cells introduced and the batch culture volume will influence cell density (line height),
and the time required to manufacture enzymes needed to complete various metabolic
processes will influence the time involved (line and phase length).
2. Exponential growth or Logarithmic growth phase – During the exponential or
logarithmic growth phase, the cells are dividing at a rapid rate and population
numbers are represented by a diagonal line extending upward from the lag phase
level. The population is metabolically very active and its numbers are increasing
exponentially (1 cell becomes 2, 2 become 4, 4 become 8, 8 become 16, 16 become 32, 32
become 64, 64 become 128, 128 become 256, etc.).
The time required for one cell to divide into two cells is called the generation time or
doubling time and for rapidly growing organisms such as E. coli, Staphylococcus or
Salmonella is typically around 20 minutes. Since an E. coli cell growing under optimum
conditions generally requires about 60 minutes to replicate its chromosome, this short
generation time seems impossible; but the bacteria accomplish this amazing feat by
overlapping their DNA replication cycles in time, i.e., before the first replication cycle
is complete, a 2nd and 3rd cycle have begun.
It has been estimated that if bacteria growing in a batch culture could be maintained in
their exponential growth phase for 45 hours, they would form a mass the size of the
earth. Needless to say, this is not possible.
3. Stationary phase - The stationary phase is represented by a horizontal line again
indicating no overall increase in cell number. During this phase the number of cells
dying is equal to the number of new cells being formed and the population has
reached its maximum density, referred to as the maximum concentration or mconcentration. For most bacteria grown in a batch culture, the m-concentration is 109
(one or more billion) cells per ml of culture medium.
During the stationary phase, some of the cells present in the population are dying due
to a lack of nutrients and a buildup of toxic metabolic waste products. Since
conditions within cells and within the environment are not uniform, the death of some
cells can provide nutrients for others not yet overcome by the toxins present, but only
for a limited time. During this phase of growth, population density is influenced by an
environmental feature called carrying capacity. Carrying capacity refers to the number
of organisms an environment can support, and is influenced by limiting factors such
as nutrient availability, pH, redox potential and temperature. The numerical value of
the carrying capacity is identical to that of the m-concentration, because population
density cannot exceed the environment's capacity to support it. All populations are
limited by the carrying capacity of their environment.
4. Exponential death phase – During the death phase, represented by a diagonal line
extending downward from the stationary level, the population is experiencing an
exponential decrease in number. Many cells are dying due to a lack of nutrients and
are being poisoned by their own metabolic waste products. The density of live cells
decreases rapidly and typically drops below the number of organisms initially
introduced into the medium. Conditions within the container are generally nasty, but
a few hardy individuals will often manage to survive; especially if metabolism is
slowed by a drop in temperature (e.g., if some conscientious individual places the
culture into a refrigerator).
If the growth curve described above were drawn without using a logarithmic scale for the
ordinate, it would form a J-shaped curve extending steeply upward until the carrying
capacity was reached, and then dropping in an equally steep downward sweep. The
growth curve representing earth's human population shows a similar J-shape. Although
the carrying capacity of this planet is not uniform and cannot be accurately predicted, it is
a feature of our environment, and cannot be indefinitely ignored. Environmental factors
such as nutrient availability, toxic waste buildup and temperature will eventually limit our
population just as they do those of microorganisms.
As a supplement to this week's lecture information, you may explore the population
Web sites listed below. Students interested can obtain 5 points extra credit by visiting
two or more of these sites and writing a brief report summarizing their content.
United Nations Population Fund - http://www.unfpa.org/index.htm
Population Connection - http://www.populationconnection.org/
The Population Institute - http://www.populationinstitute.org/
Though humans have considerable potential for intelligent achievement, our population
growth curve strongly resembles that of Escherichia coli growing in a batch culture.
Somehow it seems we should know better.
Growth in a Continuous Culture:
Microorganisms such as bacteria and yeast can be maintained in continuous cultures if
wastes (including dead cells) are removed and a steady supply of nutrients is maintained.
Continuous cultivation is often used when microorganisms are being maintained for the
synthesis of organic compounds such as enzymes, solvents, etc., or when biochemical
processes are being investigated. Population density within a continuous culture is much
more stable, and metabolic processes tend to be more consistent than are those occurring
in batch cultures.
Growth on solid media:
Bacteria grown on solid media typically form colonies of a characteristic size and shape.
Although colonies are often surrounded by what appears to be available nutrient, colony
growth is limited by the same factors limiting growth in a batch culture. Bacteria typically
release enzymes that break down nutrients beyond the colony border, and open agar
surfaces are often depleted of nutrients. Although the youngest, most viable organisms
are often found at the colony margin, cell reproduction and expansion of the colony is
essentially impossible.
Note – Motile organisms and filamentous forms can temporarily overcome nutrient
limitations by swarming (swimming over the agar surface) or by extending their filaments
into regions with available nutrients. Eventually however, the plate edge is reached, and
all available nutrients are depleted.
Students seeking to maintain pure cultures of various microorganisms must transfer
samples to new media every 2-3 weeks. Nutrient depletion and water loss will eventually
cause most of the cells on an agar plate to die, even though the colonies may appear
unchanged.